Influence of Trawl Geometry and Vertical Distribution of Fish on Sampling with Bottom Trawl

Studies were initiated at the Institute of Marine Research, Bergen, in 1984 with the aim of improving the reliability of bottom trawl and acoustic survey results for cod and haddock in the Barents Sea. Spread of the wings and doors and height of the trawl were measured during fishing operations, and acoustic recordings of fish densities were obtained. The observations revealed several aspects of geometry and performance of the gear that could reduce its efficiency for sampling different species and size-groups. The acoustic recordings indicated that fish density has a substantial influence on the correlation between estimates of abundance from trawl catches and from acoustic recordings. Furthermore, a day-night discrepancy of integrated acoustic fish abundance was indicated.